1. Field of the Invention
The present invention relates to a holographic storage device which is capable of recording and retrieving information in sub-holograms and which utilizes an illumination hologram to create an illuminating beam which is modulated by a modulator matrix to create the object beam for holographic recording.
2. Prior Art
As computers have become more and more common the requirements for both greater storage capacity and more rapid access to information have become desirable. The storage in holographic form has been proposed which makes it possible to store information in greater density than that possible with prior storage techniques, as for example magnetic storage. It is known to spatially modulate coherent light and to store the data holographically. In such storage devices, any interference fringe system can be assigned one unit of binary data or 1 bit. The presence of the fringe system corresponds to a binary 1 and its absence to the binary 0.
With the aid of holograms it is possible to project an image to arbitrary positions in space without the assistance of optical elements. This gives rise to the basis of the possibility of constructing holographic storage devices with non-mechanical, random access memories which can have very large capacities. The use of holographic storage overcomes the problems related to tolerance which are of the mechanical-optical nature such as occur in other high capacity optical storage devices since the data output is obtained by interrogating an image with a relatively crude structure located outside of the hologram.
Each holographically stored bit corresponds to a single spot or point on an object surface whose associated interference field is uniformly stored on the entire hologram surface. Holographic storage is subsequently immune to interference, since local interferences on the hologram result merely in a slight reduction in the level of freedom of interference or a reduction in the signal-to-noise ratio. The spots or points are arranged in the form of a grid pattern and if this grid pattern is coherently illuminated from the side facing away from the hologram surface, the binary numbers can be recorded in parallel form onto the hologram.
If the data stored in such a structure are to be read out in parallel, a separate photo detector will be required for each bit position. Cost considerations normally make it prohibitive to provide a sufficient number of photo detectors to enable reading out in parallel the entire contents of a large capacity storage device. To overcome this problem it has been suggested that the storage area be subdivided into small sub-holograms or subsidiary holograms whose capacity is a fraction of the overall capacity of the storage medium. With each sub-hologram having a storage capacity in the magnitude of 10.sup.3 - 10.sup.5 bits, the cost for providing a device capable of parallel read out for the entire contents of a sub-hologram is within reason. During read out, one sub-hologram or subsidiary hologram will be reconstructed at a time by being illuminated by a reproducing beam or read out beam having a cross-sectional area of the sub-hologram. The beam is directed over the entire storage surface by means of a deflecting unit so that selective subsidiary or sub-holograms can be read. In each storage medium, the capacity of the memory will be the product of the number of subsidiary or sub-holograms which number is determined by the number of separate and distinct beam directions that the deflection unit can produce, and the capacity of the sub-holograms. However, since the storage density of a non-mechanical holographic storage device is limited to a few 10.sup.4 bits per mm.sup.2, a bulk storage medium containing about 10.sup.10 bits requires a storage area of about 1 m.sup.2.
To overcome the problems of the large storage area required for storing a large number of bits, a solution is proposed in the U.S. Pat. application Ser. No. 453,922 filed Mar. 22, 1974 by Hans Eschler, Manfred Land, Gerd Goldmann, Peter Graf, Horst Kiemle and Ulrich v. Hundelshausen. In the application, the holographic storage device is provided in which data in quantities of more than 10.sup.8 bits can be read out without any difficulty. To accomplish this, the storage medium of the application is divided into a plurality of storage modules with each module consisting of a transparent body containing both a storage plate and a detector matrix. The recording of the information in each of the storage modules is accomplished in a separate recording device and then the module is mechanically transferred to the position in the holographic storage apparatus for subsequent retrieval or read out operation.